1. Field of the Invention
The inventions relate to electroluminescent lighting.
2. Brief Description of Related Developments
Electroluminescent panels are thin planar light emitting devices commonly employed to provide backlighting in a variety of the display applications, including for example laptop computers, personal digital assistants, wristwatches and a myriad of other electronic devices. One product for which electroluminescent panels are becoming increasingly prevalent is mobile phones. Generally, many of the mobile phones which are currently being produced have large backlit displays. The large displays allow a user to operate the telephone by selecting options from a displayed menu, or to receive and display data via a wireless Internet service.
Electroluminescent (EL) panels are now also being used to light up cover parts of the phone, e.g. for lighting up the rear cover for signaling or fun. Hereto, an EL panel (thin film panel) is placed under the phone cover. The EL panel typically comprises a plurality of panel regions, that can be activated separately or simultaneously. The panel regions can be different in shape and size an may form a pattern that can be used to create animations. Thus, lighting patterns can be formed that can be used to signal different information to a user.
A typical EL panel comprises a thin emission layer including an EL phosphor—a compound that emits light when excited by an electrical field—in a dielectric substrate disposed between two thin, electrically conductive electrode layers. At least one of these layers is optically transparent at the wavelength of the EL emission. By applying a high voltage across the two electrode layers, an electric field is induced between them. The phosphor is thereby excited to a higher energy state. When the electric field is removed, the atoms fall back to a lower energy state, emitting photons as visible light in the process.
Due to its structure—two conductive electrodes with a dielectric layer between them—the EL panel functionally behaves as a capacitor. The electric field induced by the applied voltage thus decreases as a charge accumulates on one of the electrodes, and correspondingly the light production of the phosphor is attenuated or the phosphor will at charge saturation conditions even cease to emit light.
Alternating current (AC) voltage must accordingly be applied to the electrodes of the EL panel to maintain continuous illumination. Typically, AC voltage in the frequency range of 300 Hz to 3000 Hz is used to drive EL panels.
In most applications, e.g. battery powered devices, the driver circuit for the EL panel is required to convert a low direct current battery voltage, such as 1.5, 3.0, 3.7 or 12 volts, into a voltage across the electrodes of the lamp that is 100-300 Volts or more, and having an alternating polarity.
Numerous circuits and systems exist within the prior art for driving EL panels from low voltage batteries. Typically, these driver circuits comprise an inverter incorporating inductors to step-up and convert the DC voltage to AC.
As previously mentioned, EL panels functionally behave as capacitors. The prior art EL panel driver circuits are designed to operate with one EL panel with a matching capacitance. As an EL panel ages, the capacitance of the lamp decreases, and without compensation, this decrease in capacitance causes the intensity of the panel to dim over time since the impendence of the drive circuit no longer matches the load.
U.S. Pat. No. 4,611,150 discloses a driver circuit including an inverter for driving an EL panel that compensates for changes in capacitance of the EL panel by voltage feedback and an oscillator being responsive to the feedback voltage to generate a time varying signal. Thus, capacitance changes caused by aging can be substantially compensated.
One of the problems with EL panels that comprise a plurality of panel regions is—as experienced by the inventors—emission intensity changes of lit panel regions when other panel regions are switched on or off. These intensity changes are perceived by observers as flaring. The panel regions can be of different size and correspondingly different capacitance. The capacitance of the load connected to the driver circuit varies therefore significantly when EL panel regions are switched on and off. The driver circuit disclosed in U.S. Pat. No. 4,611,150, that may partially reduce these undesired effects, is however complex, expensive and undesirable from an economic point of view in mass produced articles.
Another inherent problem with EL panels is that in operation, such devices emit acoustic energy within the audible range of sound waves, producing an associated objectionable buzz or hum. Specifically, the front electrode and rear electrode vibrate due to the fact that they are alternately attracted to each other (when an electric field is present) and relaxed (when the electric field is not present). Due to this vibration, air is displaced as the volume between the two electrodes is alternately compressed and then returned decompressed. This audible noise is particularly problematic in applications such as mobile phones, which in operation are held in very close proximity to the user's ear and in which any appreciable noise interferes with the use and operability of the device.
U.S. Pat. No. 5,789,870 discloses an approach to reducing the noise generated by EL panels. This patent describes a drive circuit, which predicts the transitions from positive to negative charge, and vice versa, and effectively damps these transitions. By increasing the rise and fall times of the voltage pulses applied to the electroluminescent lamp, the transitions of charge at the front and rear electrodes of the lamp are less abrupt. This in turn reduces the rate of physical deformation of the lamp electrodes resulting from opposite electrical charges, and reduces the audible output of the EL panel.
The solution described in U.S. Pat. No. 5,789,870, however, requires a complex electronic driving circuit, comprising a plurality of operational amplifiers, transistors, diodes, resistors, and capacitors. The component values of these circuit elements must be selected so as to operate properly at each desired frequency.
U.S. Pat. No. 6,563,271 discloses another approach to reduce the noise generated by EL panels. This patent describes a drive circuit for a lighting system with at least two panel regions and a driver circuit that is configured to alternately charge the electrodes of a first EL panel region while simultaneously discharging the electrodes of a second EL panel region and vice versa, preferably at the same rate. The noise produced by compression of the electrodes of the first EL panel region is thus cancelled by the noise created by the decompression of the electrodes of the second EL panel region, thereby substantially.
The solution described in U.S. Pat. No. 6,563,271, however, requires a complex electronic driving circuit, and is less useful for EL panels in which panel regions are selectively switched on and off, since the noise can only be canceled effectively if an even number of panel regions is active.